We have extended our studies on adenovirus-microtubule interactions as follows: (1) a method was devised for purifying microtubules from HeLa cells for the first time. These microtubules, in contrast to brain microtubules, contained as major non-tubulin accessory proteins, one of molecular weight 68,000, a doublet at 100,000 and one at 216,000 daltons. The in vitro binding of adenovirus to HeLa cell microtubules appeared almost as specific as the binding to chick brain microtubules (75% edge-binding compared to 85%; 50% is control or random association value). The role of the different chick and HeLa cell microtubule accessory proteins in the in vitro binding is under investigation. (2) ts-1, a mutant of adenovirus type II obtained from Dr. J. Weber (Sherbrooke University, Canada) has been shown to produce particles that do not uncoat at the nucleus. We have observed by thin-section electron microscopy that these ts-1 particles which had been grown under non-permissive conditions predominantly enter lysosomes early after infection and do not reach the cytoplasm. In a new study, we have compared the in vitro binding of different reovirus serotypes with microtubules. We found that reovirus type I and recombinants containing the sigma 1 of type I, specifically associate (81%) with microtubules in vitro, while reovirus type III does not (56%) associate with microtubules. The in vivo significance of this association is not yet clear at this time.